Biomass-derived compounds are highly functionalized, making them difficult to selectively process into a desired product because they can interact with the surface in multiple ways. Addition of oxophilic metals such as molybdenum to supported catalysts has been shown to facilitate C-O bond activation and influence selectivity for a number of deoxygenation reactions. However, the mechanism for the improvements is not known, hampering efforts to design better catalysts. Surface science techniques using single crystals can provide a fundamental understanding of the role of oxophilic metals as promoters to platinum group catalysts for deoxygenation reactions.
Temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were performed under ultrahigh vacuum on a Mo-modified Pt(111) crystal. Our recent studies have focused on how oxophilic modifiers influence oxygenate chemistry. TPD and XPS have been used to investigate surface chemistry of simple probe molecules (hydrogen, oxygen, CO, water) as well as model compounds for biorefinery streams (2-methoxyethanol, m-cresol) on unmodified and Mo-coated Pt(111) surfaces. The results indicate that modifier sites influence the adsorption energies of both oxygenates and their decomposition intermediates. For example, the desorption temperatures of CO and hydrogen were shown to decrease upon addition of Mo to the Pt surface. These UHV results have been used to help explain observed trends from complementary investigations on supported bimetallic PtMo catalysts that show improved hydrodeoxygenation (HDO) selectivity. This data indicates that the bimetallic catalysts are more selective than monometallic platinum to formation of deoxygenated products.